The cardiovascular system consists of the heart and a system of vessels for distribu­tion of the blood to the tissues of the body and to the lungs for exchange of gases.

Whether or not the blood is oxygenated, vessels that carry blood away from the heart are called arteries, and vessels that carry blood toward the heart are called veins. Circulation to the lungs (pul­monary circulation) is functionally and ana­tomically separate from circulation to the rest of the body (systemic circulation). Conceptu­ally, it is therefore useful to regard the heart as two separate pumps housed within the same organ; one is a low-pressure pump that directs blood returning from the body to the lungs (i.e., the pulmonary circulation), and the other is a high-pressure pump that distributes blood to the systemic circulation.